x² + 7x + 12 = 0
Using the factorisation method
Find two numbers such that its product gives 12 and its sum gives 7
The two numbers are 3 and 4
Replace 7 in the equation by those two numbers and then factorise
That is;
x² + 3x + 4x + 12 = 0
x(x+3) + 4(x+3) = 0
(x+3)(x+4) = 0
Either x+3 = 0 or x+4 =0
Either x = -3 or x= -4